In a touch panel input device, also known as a digitizer, an input operation is performed by using a stylus pen or finger to apply pressure in an operation area set up on the touch panel. The position in the operation area of the input operation is detected and input position data is output to a processing device, e.g., a personal computer, indicating the input operation position.
Examples of methods for detecting the input operation position include a contact method disclosed in Japanese Utility Model Number Hei 3-6731 and a resistance method disclosed in Japanese Laid-Open Patent Publication Number Hei 5-53715. In these methods, there is no clear input operation feedback, e.g., the “click” that is generated when a push-button switch is pressed, when an pressure operation is performed. The operator can only know the operation results from the processing device, e.g., a personal computer, and is therefore unsure about whether or not the input operation to the touch panel has been acknowledged.
The applicants of the present application developed a touch panel input device in which a piezoelectric substrate is secured to the touch panel so that the touch panel can be efficiently vibrated to provide pressure operation feedback to the operator without increasing the size of the entire device (see, e.g., Abstract and FIG. 1 of Japanese Laid-Open Patent Document Number 2003-122507).
FIG. 11 shows a touch panel input device 100 that uses this piezoelectric substrate 120. In this touch panel input device 100, the position of an input operation to a touch panel 101 is detected and input position data is output. A movable plate 101a and a support substrate 101a are stacked while separated by a small gap to form the touch panel 101. Conductor layers formed as uniform resistor films cover the opposing surfaces of the movable plate 101a and the support substrate 101b. By applying pressure to the movable plate 101a, the conductor layers contact each other and provide an electrical connection at the input operation position, thus allowing the detection of the presence and the position of an input operation.
The piezoelectric substrate 120 is a vibrating element that vibrates when the touch panel input device 100 detects an input operation. The piezoelectric substrate 120 vibrates the touch panel 101 by expanding and contracting itself, thus indicating to the operator, via the finger that is touching the touch panel 101, that an input operation has been acknowledged. As shown in FIG. 11, the piezoelectric substrate 120, formed as a long, thin strip, is secured to the back surface of the movable plate 101a. A drive potential output from an AC (alternating current) power supply circuit 110 shown in FIG. 12 is applied to drive electrodes 120a, 120b formed on the front and back surfaces of the piezoelectric substrate 120. When a drive potential in the form of an AC potential waveform of approximately ±100 V (volts) is applied to the drive electrodes 120a, 120b, the piezoelectric substrate 120 flexes in the direction of the thickness axis, indicated by the arrows in FIG. 12, and vibrates the secured touch panel 101 with an amplitude large enough to provide adequate feedback to the finger performing the input operation.
A drive potential in the form of an AC potential waveform of approximately ±100 V must be provided to expand and contract the piezoelectric substrate 120, but the touch panel input device 100 may be installed in a portable device, e.g., a notebook computer, that uses a low-potential DC (direct current) power supply of approximately 5 V. For this reason, the AC power supply circuit 110 requires a step-up circuit to increase the low-potential DC power to a potential of approximately ±100 V and a wave-shaping circuit that shapes a DC potential waveform to an AC potential waveform needed to drive the piezoelectric substrate 120.
FIG. 12 is a block diagram of the AC power supply circuit 110 (see, e.g., sections 0081-0085 and FIG. 7 of Japanese Laid-Open Patent Publication Number 2004-21697). In the AC power supply circuit 110 shown in FIG. 12, a step-up oscillator circuit 111 generates an oscillation of 20 -30 kHz (kilohertz) with a constant-potential DC power supply. A step-up circuit 112 switches the current that flows to a transformer using the cycle of the step-up oscillator circuit 111. The constant-potential DC power of a few volts is stepped up to a DC potential of approximately 100 V and is sent to an amplifier circuit 113.
A vibration oscillator circuit 114 generates a drive signal with a frequency for operating the piezoelectric substrate 120 which is output to the amplifier circuit 113. The amplifier circuit 113 amplifies the drive signal using the DC potential received from the step-up circuit 112 and sends the result to a gate circuit 115.
A pulse width generator circuit 116 is connected to the input side of the gate circuit 115 to generate pulses with time widths for vibrating the piezoelectric substrate 120 when a trigger for vibrating the piezoelectric substrate 120 is received. When this pulse is being received, the gate circuit 115 sends the drive signal received from the amplifier circuit 113 to the drive electrodes 120a, 120b of the piezoelectric substrate 120 to serve as the drive potential.
With this AC power supply 110, the frequency of the drive signal generated by the vibration oscillator circuit 104 and the pulse width generated by the pulse width generator circuit 106 can be set up as desired so that the piezoelectric substrate 120 can be expanded and contracted at different frequencies and intervals. Thus, the touch panel 101 generates vibrations with different sensations for different purposes.
The piezoelectric substrate 120 used as the vibration source in the touch panel 101 is formed as a single-layer substrate made from a piezoelectric material such as a piezoelectric ceramic. The electrostriction properties of this type of ferroelectric body is used to generate warping in the piezoelectric substrate 120, but if a DC field is applied for an extended period, degradation can take place and the piezoelectric constant can decrease. This results in inadequate warping, i.e., vibration in the touch panel 101, even when an AC drive potential is applied.
Also, since the AC power supply circuit 110 is connected to the drive electrodes 120a, 120b of the piezoelectric substrate 120, during the period before the touch panel input device 100 is shipped and during times when the touch panel input device 100 is not being operated, a charge potential stored in the capacitors and stray capacitance in the AC power supply circuit 110 is applied to the drive electrodes 120a, 120b, resulting in the degradation of the piezoelectric substrate 120 due to a DC electric field as described above.
One possible solution is the addition of a selector switch to the conventional touch panel input device 100 that disconnects the piezoelectric substrate 120 from the AC power supply circuit 110 when the touch panel input device 100 is not in use. However, this does not completely solve the problem described above since there is the possibility of a statically charged conductor coming into contact with one of the drive electrodes 120a, 120b of the piezoelectric substrate 120 or with a drive power supply circuit pattern connected to one of the electrodes.
The present invention overcomes the problems of the conventional technology described above and provides a touch panel input device that reliably prevents degradation of the piezoelectric substrate. Also, in the touch panel input device of the present invention, the drive potential generated by the AC power supply circuit is doubled and applied to the piezoelectric substrate to generate a vibration with a greater amplitude while reliably preventing degradation of the piezoelectric substrate when the device is not operating.